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This example shows how to define and use views in ODB. It includes views
that show how to load a subset of data members from objects or columns
from tables, perform aggregate queries, and join multiple objects and
tables using object relationships and custom join conditions.
The example consists of the following files:
employee.hxx
Header file defining the 'country', 'employer', and 'employee' persistent
classes. The example also uses the "legacy" 'employee_extra' table that is
not mapped to a persistent class.
After the persistent classes, this header defines a number of views that
show how to obtain various information from the above object model.
employee-odb.hxx
employee-odb.ixx
employee-odb.cxx
employee.sql
The first three files contain the database support code and the last file
contains the database schema for the employee.hxx header.
These files are generated by the ODB compiler from employee.hxx using the
following command line:
odb --std c++11 -d <database> --generate-schema --generate-query \
--default-pointer std::shared_ptr --generate-session employee.hxx
Where <database> stands for the database system we are using, for example,
'pgsql'.
The --default-pointer option is used to make std::shared_ptr the default
object pointer. The --generate-session option is used to enable session
support for all the objects which is required to use object loading views.
database.hxx
Contains the create_database() function which instantiates the concrete
database class corresponding to the database system we are using.
driver.cxx
Driver for the example. It includes the employee.hxx and employee-odb.hxx
headers to gain access to the persistent classes and views as well as
their database support code. It also includes database.hxx for the
create_database() function declaration.
In main() the driver first calls create_database() to obtain the database
instance. It then creates the legacy 'employee_extra' table and proceeds
to populate the database with a number of 'country', 'employer', and
'employee' objects. Once this is done, the driver uses views defined in
employee.hxx to load and print various information about the object model.
To compile and link the example manually from the command line we can use the
following commands (using PostgreSQL as an example; replace 'c++' with your
C++ compiler name):
c++ -c employee-odb.cxx
c++ -DDATABASE_PGSQL -c driver.cxx
c++ -o driver driver.o employee-odb.o -lodb-pgsql -lodb
To run the example we may first need to create the database schema (for some
database systems, such as SQLite, the schema is embedded into the generated
code which makes this step unnecessary). Using PostgreSQL as an example, this
can be achieved with the following command:
psql --username=odb_test --dbname=odb_test -f employee.sql
Here we use 'odb_test' as the database login and also 'odb_test' as the
database name.
Once the database schema is ready, we can run the example (using PostgreSQL as
the database):
./driver --user odb_test --database odb_test
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